1. Field of the Invention
The present invention relates generally to the field of telecommunications. More particularly, the present invention relates to the field of linefeed control for subscriber loops.
2. Description of Related Art
Subscriber line (or loop) interface circuits (SLICs) are typically found in or near the central office exchange of a telecommunications network.
One typical SLIC provides a communications interface between a digital switching network for a central office exchange and an analog subscriber line. The analog subscriber line connects to subscriber equipment, such as a subscriber station or telephonic instrument for example, at a location remote from the central office exchange. The analog subscriber line and subscriber equipment form a subscriber loop.
The SLIC detects and transforms voiceband communications transmitted from the subscriber equipment in the form of low voltage analog signals on the subscriber loop into corresponding digital data for transmission to the digital switching network. For bi-directional communication, the SLIC also transforms digital data received from the digital switching network into corresponding low voltage analog signals for transmission on the subscriber loop to the subscriber equipment.
The SLIC also typically generates a linefeed on the subscriber loop to power the subscriber equipment. When a conventional telephone, for example, on the subscriber loop is off-hook, the SLIC should provide adequate direct current (DC) feed on the subscriber loop to help ensure proper operation of the telephone. When more than one telephone is off-hook on the same subscriber loop, however, one or more of the off-hook telephones may not receive adequate power.
The SLIC may be designed to provide adequate power to accommodate a larger load on the subscriber loop. The SLIC, however, would then unnecessarily waste power whenever a smaller load, such as only one off-hook telephone for example, is on the subscriber loop. Also, the SLIC would have to be designed to dissipate a maximum possible amount of wasted power to account for times when only a smaller load is on the subscriber loop. This maximum possible amount of wasted power typically increases as the load the SLIC is designed to support increases.
The SLIC may alternatively be designed to provide a resistive DC feed on the subscriber loop so that the power supplied to the subscriber equipment on the subscriber loop is generally proportional to the DC load on the subscriber loop. As additional telephones, for example, are switched off-hook on the same subscriber loop, the increasing DC load presented on the subscriber loop will draw from the SLIC an increasing amount of power generally in proportion to the DC load. Providing a typical resistive DC feed on a subscriber loop, however, generally elevates power dissipation for all DC load conditions and perturbs voice impedance synthesis performed by the SLIC.